A VCSEL is a semiconductor laser including an active region sandwiched between mirror stacks that can be semiconductor distributed Bragg reflectors (DBRs) [N. M. Margalit et al., “Laterally Oxidized Long Wavelength CW Vertical cavity Lasers”, Appl. Phys. Lett., 69 (4), Jul. 22 1996, pp. 471-472], or a combination of semiconductor and dielectric DBRs [Y. Oshio et al., “1.55 μm Vertical-Cavity Surface-Emitting Lasers with Wafer-Fused InGaAsP/InP—GaAs/AlAs DBRs”, Electronics Letters, Vol. 32, No. 16, 1st Aug. 1996]. One of the mirror stacks is typically partially reflective so as to pass a portion of the coherent light that builds up in a resonating cavity formed by the mirror stacks sandwiching the active region. The VCSEL is driven by a current forced through the active region. Mirror stacks are typically formed of multiple pairs of layers formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. For example, a GaAs based VCSEL typically uses an AlAs/GaAs or AlGaAs/AlAs material system wherein the different refractive index of each layer of a pair is achieved by altering the aluminum content in the layers.
VCSELs are well adapted as preferred light sources for communication applications, due to the following advantageous features: a single mode signal from a VCSEL is easily coupled into an optical fiber, has low divergence, and is inherently single frequency in operation.
One of important requirements for the operation of a VCSEL is to compensate for the small amount of gain media which is typical for VCSELs due to the compact nature thereof. This is associated with the fact that, in order to reach the threshold for lasing, the total gain of a VCSEL must be equal to the total optical loss of the VCSEL. To compensate for the small amount of gain media, and to enable reaching and maintaining the lasing threshold, it is known to use wafer fusion of one or both of the mirror stacks, with reflectivity values exceeding 99.5%, to the active region. Wafer fusion is a process by which materials of different lattice constant are atomically joined by applying pressure and heat to create a real physical bond.
VCSELs emitting light having a long wavelength are of great interest in the optical telecommunication industry. A long wavelength VCSEL can be obtained by using a VCSEL having an InGaAs/InGaAsP active cavity material, in which case an InP/InGaAsP material system must be used for the mirror stacks in order to achieve a lattice match to the InP. In this system, however, it is practically Impossible to achieve DBR based mirrors with-high enough reflectivity because of the small difference in the refractive indices in this material system. Many attempts have been made to address this problem including a wafer fusion technique in which a DBR mirror is grown on a separate substrate and fused to the active region.
Another important requirement for fundamental mode operation of a VCSEL and light coupling into a single mode fiber, is current and optical confinement. In order to reduce the light emitting area of the VCSEL (practically to about 5-10 μm), the opening of current flow (current aperture) is restricted through lateral oxidation of Al-containing layers which also creates a lateral refractive index variation for fundamental optical mode operation of these devices. In such a lateral oxidation technique, a mesa is etched into the top surface of the VCSEL wafer, and the exposed sidewalls of an Al-containing layer (typically AlGaAs layer) are exposed to water vapor at elevated temperature. Water vapor exposure causes conversion of the AlGaAs to AlGaOx, some distance in from the sidewall toward the central vertical axis depending on the duration of oxidation. Formation of the current aperture defines the active region of the device which includes the active cavity material where there is a current flow and the light is generated, while lateral refractive index variation allows to control the mode structure of the emitted light. This approach has been used for practically all short-wavelength AlGaAs/Ga(In)As(P) VCSELs (i.e., emitting at 0.65-1.1 μm) and is also applied to long wavelength VCSELs (i.e., emitting at 1.25-1.65 μm) that may comprise DBR mirrors grown in the same material system as the active region [S. Rapp et al., “Near-Room-Temperature Continuous-Wave Operation of Electrically Pumped 1.55 μm Vertical cavity Lasers with InGaAsP/InP Bottom Mirror”, Electronic Letters, Vol. 35, No. 1, 7th Jan. 1999], and AlGaAs based DBRs that are as-grown [W. Yuen et al., “High Performance 1.6 μm Single-Epitaxy Top-Emitting VCSEL, Electronic Letters, Vol. 36, No. 13, 22th Jun. 2000] or wafer-fused on the active cavity material grown on InP (as in the above-indicated article of N. M. Margalit et al.). However, this approach leads to a non-planar structure, since mesa etches are required, resulting in a complicated processing scheme and low yield. The lateral oxidation is very sensitive to various factors like temperature, surface quality and defects, and does not allow obtaining current apertures with a precise size, and, above all, uniform enough to be used in the fabrication of multiple wavelength arrays by cavity length engineering. In case of lateral oxidized devices, it is quite difficult to use high performance AlAs/GaAs DBRs with highest refractive index contrast and best thermal characteristics, as compared to other AlGaAs/GaAs DBRs.
The use of the wafer fusion technique allows for obtaining both current and optical confinement during the fusion of the p-type GaAs-based DBRs to the p-side of the active cavity material grown on InP wafers. To this end, a special structuring of one of two contacting wafers is performed. The structured surface consists of a central mesa surrounded by shallow etched regions and a large area of unetched semiconductor. The fusion front in the central mesa and the large area of the unetched semiconductor are in the same plane. The current confinement is obtained by placing a native oxide layer at the fused interface outside the central mesa [A. V. Syrbu, V. P. Iakovlev, C. A. Berseth, O. Dahaese, A. Rudra, E. Kapon, J. Jacquet, J. Boucart, C. Stark, F. Gaborit, I. Sagnes, J. C. Harmand and R. Raj, “30° CW Operation of 1.52 μm InGaAsP/AlGaAs Vertical Cavity Lasers with in situ built-in lateral current confinement by localized fusion”, Electronic Letters, Vol. 34, No. 18, 3rd Sep. 1998; or by placing a proton implanted region at the fused interface outside the central mesa (U.S. Pat. No. 5,985,686). This approach, however, suffers from the following drawbacks: the fused p-GaAs-based and p-InP-based interfaces are normally highly resistive resulting in a substantial heating of the device; and it is very difficult to optimize p-AlGaAs/GaAs DBRs for long wavelength VCSELs to have both high reflectivity (low absorption) and low resistivity.
According to a different approach of the long wavelength VCSEL fabrication technique, tunnel junctions can be used to inject holes into the active region, allowing using n-type DBRs on both sides of the active cavity material. In U.S. patent WO 98/07218, the p-side of a InP-based active cavity material is fused to the p-GaAs side of an AlGaAs/GaAs based structure including the n-type DBR stack and the n++/p++ tunnel junction. Standard mesa etching and AlGaAs wet oxidation are performed for lateral optical and current confinement in these devices. Besides the above-mentioned drawbacks related to this particular lateral confinement technique and to highly resistive p-GaAs/p-InP fused junctions, this approach suffers from the known difficulty in obtaining low resistive reversed biased tunnel junctions in GaAs, as compared to lower band-gap materials.
In a more recent approach, the so-called “buried tunnel junction structure” formed in the low band-gap InP-based active cavity material is used. [M. Ortsiefer et al., “Room-Temperature Operation of Index-Guided 1.55 μm In-P-based Vertical cavity Surface-Emitting Laser”, Electronic Letters, Vol. 36, No. 13, 2nd Mar. 2000]. This VCSEL structure comprises one oxide DBR and one semiconductor DBR. The n-type semiconductor DBR, the cavity material terminating with a p-type material, and the p++/n++ tunnel junction structure are grown in the first epitaxial process. Then, a shallow mesa structure is etched through the tunnel junction until reaching the p++ region and regrown with a n-type InP layer in the second epitaxial process. This is followed by the deposition of an oxide DBR on the n-InP. In this structure, the buried tunnel junction provides a means for lateral current confinement. However, an oxide DBR with intrinsically low thermal conductivity is placed between the active region and the heat-sink. The final device represents a free standing epitaxial structure without a substrate, thereby adding complexity in handling and processing such devices and reducing the yield.
The article “Metamorphic DBR and Tunnel-Junction Injection: A CW RT Monolithic Long-Wavelength VCSEL”, J. Boucart et al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 3, 1999, pp. 520-529 discloses a VCSEL comprising a tunnel junction incorporated into the active cavity material and a metamorphic n-type AlGaAs/GaAs DBR grown over the active cavity material in the same epitaxial process. In this case, the heat dissipation is improved due to the good thermal conductivity of the n-AlGaAs DBR. The lateral current confinement is obtained as a result of deep proton implantation through the top AlGaAs/GaAs DBR and tunnel junction. However, such a structure is characterized by the lattice mismatch of 3.7% between GaAs-based and InP-based compounds, resulting in a high density of defects in the metamorphic AlGaAs/GaAs DBR. These defects propagate into the active region which may result in a fast degradation of the device. The proton implantation also creates defects and especially in the InP-based active cavity material. Additionally, the resulting structure does not comprise a means for lateral optical confinement.
New generations of local area networks will use the wavelength division multiplexing (WDM) concept in order to achieve broad band transmission. Multiple wavelength VCSEL arrays may play an important role in these systems. The article “WDM Array Using Long-Wavelength Vertical Cavity Lasers” V. Jayaraman and M. Kilcoyne in Proc. SPIE: Wavelength Division Multiplexing Components, vol. 2690, 1996, pp. 325-336, discloses optically pumped VCSEL arrays emitting at 1550 nm in which cavity length of different VCSELs in array is changed by selective etching of an InGaAsP/InP superlattice which is included in the VCSEL cavity. The disadvantage of this device structure is that it also does not include a means for lateral optical confinement.